What is facultative anaerobe?

A facultative anaerobe is an organism that survives with or without molecular oxygen. It is the most adaptable type of organism and can live in both, aerobic as well as anaerobic environments. 

Facultative anaerobes are capable of switching between aerobic and anaerobic modes of respiration in order to generate energy. When oxygen is available, they switch to aerobic respiration, which is the preferred mode as it is more efficient. In oxygen-deficient environments, they switch over to anaerobic respiration to derive energy for survival.